The present invention relates generally to route guidance systems for use in automobiles, and more specifically to methods and apparatus for determining vehicle position relative to a preselected route in such route guidance systems.
Known route guidance systems have a limited capacity for tracking the position of a vehicle relative to a map stored in a database. Frequently, such systems calculate vehicle position using dead-reckoning, wherein the vehicle's current position is determined by considering a known previous position of the vehicle and the direction and distance travelled since the previous position. In such dead-reckoning systems, it is necessary to measure the heading of the vehicle relative to a reference direction, as well as the distance travelled from a particular point. In order to measure vehicle heading, route guidance systems typically utilize a magnetic compass, as well as a gyroscope or other relative heading sensor. An odometer is typically used to measure distance travelled.
It has been found, however, that, for a variety of reasons, such dead reckoning systems suffer from significant error in determining vehicle position. This positioning error has been found to accumulate as the vehicle progresses. Factors contributing to this error include the measurement error associated with the heading and distance sensors, as well as erroneous measurements caused by, for example, local magnetic fields, environmental condition fluctuation and improper calibration of the sensors. Thus, while an ideal system might accurately determine vehicle position by merely comparing the measured position to the database to find a matching position on the map, in actuality, route guidance systems must compensate for the accumulating error in measured vehicle position to maintain a reasonable level of accuracy.
Where a measured position fails to correspond with a position on the route or on the map, route guidance systems must decide whether the discrepancy is due to an actual divergence of the vehicle from the roadways on the route or map, or whether the discrepancy is due merely to error in the measured position or error in the digitized data of the database. While a measured position lying a few feet off of a roadway on the map is most likely to be caused by measurement or database error, it is conceivable that the vehicle has pulled off of the roadway into a driveway or other place not part of the map database. Further, when a vehicle passes through a fork in a road, or is travelling along one of two parallel roads very close to one another, the inability to precisely measure vehicle position creates difficulty in determining on which of the possible roads the vehicle is actually located.
A number of known route guidance systems simply avoid this problem by assuming the vehicle is always on a road on the map, or always on the selected route. This simplifies the route guidance process to merely choosing the most likely road on which the vehicle is travelling, disregarding any indication that the measured position appears off-road or off-route.
In more sophisticated route guidance systems, vehicle position is tracked by calculating an error region surrounding the dead-reckoned position in which there is a high probability that the actual vehicle position will be located. The characteristics of the error region are typically a function of the accuracy of the sensors and the database, the region usually being determined based upon empirical data gathered through system testing.
In such systems, all road segments in the database falling within the error region are flagged as potential current positions of the vehicle, although some may be immediately excluded based on such factors as heading variation or lack of connectivity to a previous position. All or a selected portion of these position possibilities are then tracked to help determine which position possibilities generated at later points are likely to correspond to actual vehicle position, based on such factors as distance from the dead reckoned position, heading, connectivity, closeness of road segments on which the positions lie, and correlation in the shape of road segments to the path of vehicle travel. An exemplary vehicle positioning system using such an error region is seen in U.S. Pat. No. 4,796,191 entitled "Vehicle Navigational System and Method", the full disclosure of which is incorporated herein by reference.
During the time that this tracking of the error region is occurring, however, systems designed to provide route guidance must periodically notify the driver of whether s/he is still on the route, what maneuver is required next, whether the vehicle has arrived at the destination, and other such information. Such systems must therefore determine which of the plurality of position possibilities generated in the error region is the best choice for purposes of providing a position update to the driver. Typically, many of the position possibilities generated in the error region for purposes of tracking vehicle position are not useful in determining current position for purposes of providing route guidance updates to the driver. It is therefore necessary to filter the position possibilities generated for purposes of tracking vehicle position to remove those less useful for route guidance, and make a best estimate of a current position of the vehicle from the remaining position possibilities.
In light of the uncertainty associated with selecting a current position for route guidance purposes, a degree of error in the selection process is expected. Even where a vehicle is travelling continuously on roads on the route, in certain situations a system may tend to select on-route and off-route positions as close as seconds apart. Such systems will then communicate to the driver the changing status of vehicle position from on-road to off-road and back again, even though the vehicle has never left the route. Such fluctuations can confuse the driver and result in failure of the system to notify the driver of necessary maneuvers, arrival, road conditions and other route guidance information.
For these and other reasons, a route guidance system is desired which provides more accurate determinations of current position for purposes of providing route guidance updates. The system should be capable of compensating for the measurement error inherent in the heading and distance sensors, as well as in the digitized data of the map databases commonly used in vehicle navigation systems. Preferably, the method and apparatus should be tolerant of error within a given limit, such that the system maintains continuity in the route guidance information provided to the user. The method and apparatus should be capable of determining whether the vehicle position is on or off a preselected route. Further, the system and method should preferably communicate route information, upcoming maneuvers, arrival notification and other such information to the driver of the vehicle.